An immune response can be induced against an almost limitless variety of substances. There are two categories of immune response, humoral and cellular. In a humoral response, the antigen or foreign substance is recognized by specific receptors on the surfaces of lymphocytes and certain B-lymphocytes are stimulated to multiply. These cells produce vast numbers of antibodies capable of binding to the antigens. After binding, the bound antigen is removed or destroyed. In a cellular or cell-mediated immune response, T-lymphocytes, in particular helper and killer T-lymphocytes, are active in recognizing and destroying the foreign substance.
The process of inducing immunity as a preventive measure against infectious diseases is referred to as immunization. Immunization can be passive, where antibodies are administered to an animal to provide a short-term immunity, or active. Active immunization is more commonly known as vaccination, where a killed or weakened antigen, or fragment thereof, is introduced to an animal. The antigens sensitize the immune system so that if that antigen later enters the body, antibodies are quickly produced to remove and/or destroy the invading antigen.
Most vaccines are preparations containing the organism, or parts of organism, against which protection is sought. The organism is killed or weakened sufficiently so as not to cause disease but to induce immunity. Some vaccines contain chemically modified bacterial toxins. Vaccines against a variety of infectious diseases are available, for example, live attenuated vaccines are given to protect against measles, mumps, rubella, yellow fever and polio. Diphtheria and tetanus vaccines contain inactivated bacterial toxins. Cholera, typhoid fever, pertussis, rabies, viral hepatitis B and influenza contain killed organisms, or in the case of hepatitis B, only a part of the virus.
Vaccines are usually given by injection into the upper arm, or in the case of polio vaccine, administered orally. Administration via injection is not always convenient, particularly in third world countries, for soldiers in the field or for subjects other than humans, including horses, cows and dogs.